In an exhaust gas purification system for an internal combustion engine, an exhaust gas purification apparatus such as an NOx storage reduction catalyst (which will be hereinafter referred to as an “NOx catalyst”), a particulate filter on which a catalyst is supported (which will be hereinafter referred to as a “filter”), and a combination of them is provided in some cases in the exhaust passage of an internal combustion engine. There are also cases where a precatalyst having an oxidizing ability is provided in the exhaust passage upstream of the exhaust gas purification apparatus, and a reducing agent addition valve is provided in the exhaust passage immediately upstream of the precatalyst.
In this case, when the temperature of the exhaust gas purification apparatus is to be raised or the air-fuel ratio of the ambient atmosphere around the exhaust gas purification apparatus is to be decreased in order to recover the function of the exhaust gas purification apparatus, a reducing agent is supplied to the precatalyst and the exhaust gas purification apparatus by adding the reducing agent through the reducing agent addition valve.
In cases where a catalyst having a heat capacity smaller than that of the exhaust gas purification apparatus is used as the precatalyst, adding the reducing agent for supply to the precatalyst enables a temperature rise of the precatalyst at an earlier time, for example, during the cold start of the internal combustion engine. In consequence, the temperature of the exhaust gas purification apparatus can be raised at an earlier time.
Japanese Patent Application Laid-Open No. 2007-032398 describes a technology in which a fuel addition valve and a combustion catalyst are provided in a branch passage that branches from an exhaust passage at a position upstream of the exhaust gas purification apparatus to lead to the exhaust gas purification apparatus. According to this Japanese Patent Application Laid-Open No. 2007-032398, an air pump and an ignition plug are further provided in the branch passage.
Japanese Patent Application Laid-Open No. 2005-127257 describes a technology in which a reforming catalyst that reforms supplied fuel is provided in an exhaust passage upstream of an NOx catalyst. According to the technology described in this Japanese Patent Application Laid-Open No. 2005-127257, the reforming catalyst is disposed at a central position of the exhaust passage so that a roundabout gas flow channel is formed around the outer circumference of the reforming catalyst.
Japanese Patent Application Laid-Open No. 2006-70818 describes a technology in which a fuel injector is provided in an exhaust passage, and fuel is injected from the fuel injector, at a time when the temperature of the exhaust gas is equal to or higher than a reference temperature and the operation state of the internal combustion engine is an accelerating state, thereby controlling the temperature of the exhaust gas.
Japanese Patent Application Laid-Open No. 2006-275020 discloses a technology in which to prevent an excessive temperature rise of an exhaust gas purification catalyst from occurring, nitrogen-enriched air is supplied to an exhaust passage upstream of the exhaust gas purification catalyst.
Japanese Patent Application Laid-Open No. 8-158928 discloses a technology pertaining to a method of calculating the exhaust gas temperature taking into account the heat of vaporization of condensed water. Japanese Patent Application Laid-Open No. 2005-163586 discloses a technology pertaining to a method of raising the temperature of an NOx catalyst using an HC adsorption catalyst.